One approach is to express the toll participation is causing.
Tell them what it feels like to have learned the religious history you'd accepted is different from what you learned when looking up that very history as recorded by the religion itself. For example, recently a GB member said that JWs back in 1914 got it right about what occurred invisibly in the year 1914. Nothing could be further from the truth. What JWs (Bible Students) believed and published about 1914 during and before 1914 is nothing like what is taught today about 1914. Then there are all the unanswered questions about how its scriptural to shun JWs for accepting transfusion of one product rendered from blood but not proper to shun a JW for accepting another product rendered from blood. Then there's the religion's history of failing to have an adequate policy to protect children from pedophiles. Then there's the religion's history of lambasting other religious movements when they make false predictions, yet fail to be as candid about their own false predictions.
Unless a person wants to live a life intellectually hiding under rocks there is no way to intelligently overlook these things. Oh, and don't forget to mention what it feels like to be made to feel some questions are best left unasked, even when answers are essential to an important biblical position. It's unhealthy to live like this, that is unless a person gets comfort from being an intellectual cave-dweller.
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